The present invention relates to a ferroelectric memory device and a method of manufacturing the same.
A nonvolatile memory device using spontaneous polarization specific to a ferroelectric (ferroelectric memory device) has attracted attention as an ultimate memory having the possibility of replacing not only a conventional nonvolatile memory but also most memories such as a static RAM (SRAM) and a DRAM due to its characteristics such as a high-speed write/read and low-voltage operation. As the ferroelectric material, a number of candidate materials have been proposed. In particular, a perovskite-type oxide such as lead zirconate titanate (Pb(Zr,Ti)O3, hereinafter abbreviated as “PZT”) and a bismuth-layered compound such as SrBi2Ta2O9 are considered to be promising materials due to their extremely excellent ferroelectric characteristics.
In general, when using the above-mentioned oxide material as a capacitor insulating layer, an interlayer dielectric such as SiO2 is deposited after forming an upper electrode in order to provide electrical insulation between each memory element as the major objective. As the deposition method, a chemical vapor deposition (CVD) method excelling in step coverage is generally used. However, use of such a deposition method causes hydrogen to be generated as a reaction by-product. In particular, if activated hydrogen passes through SiO2 and the upper electrode and reaches the ferroelectric thin film, the crystallinity of the ferroelectric is impaired due to the reducing effect of hydrogen, whereby the electrical characteristics deteriorate to a considerable extent. The characteristics of a MOS transistor as a switching element deteriorate due to lattice defects occurring in the silicon single crystal during the device manufacturing step. Therefore, it is necessary to subject the MOS transistor to a heat treatment in a hydrogen-containing nitrogen gas in the final stage. However, since the hydrogen concentration in this step is higher than that during formation of the interlayer dielectric, damage occurring to the ferroelectric thin film is more acute.
In order to prevent the reduction and deterioration of the ferroelectric capacitor due to hydrogen, a method of depositing a protective film after forming a ferroelectric thin-film capacitor so as to cover the ferroelectric thin-film capacitor to prevent entrance of hydrogen has been attempted. This protective film is generally called a hydrogen barrier film. Since the ferroelectric capacitor is isolated from the hydrogen atmosphere during formation of the interlayer dielectric due to the presence of the protective film, deterioration of the electrical characteristics from the initial value can be prevented.
The hydrogen barrier film is generally deposited after forming the ferroelectric capacitor so as to cover the ferroelectric capacitor. This enables the ferroelectric capacitor to be isolated from hydrogen generated in the subsequent step. However, if the ferroelectric capacitor is covered with the hydrogen barrier, elements other than hydrogen are also intercepted. For example, oxygen is not supplied to the ferroelectric even if the substrate is heated in an oxygen atmosphere, since the hydrogen barrier film also functions as an oxygen barrier film. Specifically, once the ferroelectric capacitor is covered with the hydrogen barrier film, it is extremely difficult to recover the crystallinity of the ferroelectric to secure the electrical characteristics of the capacitor. In the stage in which the hydrogen barrier film is applied, it is indispensable that the ferroelectric capacitor maintain desired characteristics. However, damage to the ferroelectric often occurs during formation of the ferroelectric capacitor. In general, processing using dry etching is employed for forming the capacitor, and the resist must be removed after completion of the objective etching. As the removal method, a method of burning the resist by exposure to O2 plasma or N2 plasma can be given. However, if hydrogen generated during the resist combustion process, a water molecule, or a reducing etching gas passes through the upper electrode and reaches the interface between the upper electrode and the ferroelectric, the ferroelectric is reduced in this region, whereby the crystallinity is impaired to a considerable extent. Since such damage cannot be sufficiently corrected by subsequent heating and is more sensitive to the reducing atmosphere in the subsequent step, the crystallinity is easily disordered. Therefore, a damaged region which cannot contribute to polarization switching occurs.
If the hydrogen barrier film having a tensile stress is provided on the ferroelectric capacitor, the loss of polarization electric charge of the ferroelectric is increased on the sidewall of the capacitor. In particular, a minute capacitor cannot exhibit sufficient characteristics.